Project Page

http://www.SignalONE.com/radioastronomy/telescope/

5.2 Meter Radio
Astronomy Project for 1420 MHz

ELEVATION
POSITIONING SYSTEM

The elevation of the antenna is measured using a
digital inclinometer. A digital inclinometer is a small
device which contains a sensor and additional miniture
electronics to convert its mechanical position into
digital data which is sent to a serial port of the
antenna tracking computer.

DIGITAL INCLINOMETER:
The inclinometer I used is available from SmartTool
Technologies of 1717 Grant St., Santa Clara, CA 95050
(408-653-1680) as their "ISU Circuit Board - Part
No. 90104001". Its intended use is as a component of
a digital carpenter's level, and the cost is about $100.
It is built on a small circuit board measuring 1.3 x 2.6
inches and requires 5 volts at about 2 milliamps for
power. In addition to the power lead, there are
connections for receive data (RX), transmit data (TX),
and a common ground. When the inclinometer receives the
proper 5-byte digital command it responds with a 6-byte
message containing the elevation information accurate to
about 0.1 to 0.2 degrees.

SPECIFICATIONS: The
following links will provide copies of the SmartTool
inclinometer spec sheet:

MECHANICAL INSTALLATION:

The inclinometer needs to be mounted securely to
the antenna in a weatherproof container large enough for
the inclinometer as well as a TTL to RS-232 interface
(see below). The actual sensor element is a disk about
1.25 inches in diameter and 0.25 inches thick. The plane
of the sensor must be mounted perpendicular to the
elevation axis of the antenna.

ELECTRICAL INTERFACE:

The digital inclinometer operates at 9600 baud
(no parity, 8 data bits, 1 stop bit), TTL logic levels of
0 and +5 volts. A small additional circuit is required to
convert the TTL logic levels to the RS-232 levels
required by the computer serial port. You can build your
own (a circuit diagram is included in the application
notes provided with the inclinometer, see below) or you
can use a commercially available equivalent
as I did. The interface circuits typically require 9 to
12 volts D.C. I supplied 12 volts to the interface
circuit and obtained the 5 volts required by the
inclinometer with a 5-volt regulator fed from the same
source. There are four wires required to operate the
inclinometer/level converter: +12 volts, ground, RS-232
transmit data, and RS-232 receive data. The two RS-232
lines and ground connect to the serial port of the
computer (COM1, COM2, etc.) and regulated 12 volts D.C.
is supplied to the 12-volt line (and ground) from a
separate source.

CLS
OPEN "COM1:9600,N,8,1,ASC,RS,RB4096" FOR RANDOM
AS #1

start:
PRINT #1, CHR$(3) + CHR$(4) + CHR$(88) + CHR$(2) +
CHR$(94)

a$ = INPUT$(6, #1)

FOR n = 1 TO 6
b(n) = ASC(MID$(a$, n, 1))
NEXT n

angle = ((256 * b(4)) + b(5)) * 360 / 65536

el = 90 - angle
corr = 3
el = el + corr

LOCATE 1, 1: PRINT USING "####.##"; el
GOTO start

END

FACTORY SUPPLIED SOFTWARE:

Included with the inclinometer is a computer disk
containing a utility program "ST.EXE."
This program can be used to test the inclinometer and has
a number of other features, including the ability to read
and write to the inclinometer's read-only memory (ROM).
During manufacture, the inclinometers are
factory-calibrated and linearized. The calibration data
is stored in an on-board ROM. It is possible to corrupt
the contents of the ROM when experimenting with hardware
and software for the device, thereby rendering it
unusable. Fortunately, the program ST.EXE allows the user
to save the initial contents of the ROM to disk and
restore them if corruption should occur. The user must be
sure to save the contents of the ROM immediately upon
establishing serial communication with the device! The
following links provide additional information about the
supplied software:

COARSE CALIBRATION:

Calibration of the inclinometer is done by simply
adding or subtracting a fixed amount from the reading
provided by the above program. In the program above, a 90
degree adjustment is first made to the angle which
compensates for the fact that my inclinometer happened to
be mounted with its reference direction at roughly a
right angle to the horizon. In fact, the mounting
position is arbitrary as any position can be accomodated
by adding or subtracting the correct amount.
Additionally, a 3-degree rough correction factor was
needed for my installation. This is equivalent to
mechanically rotating the inclinometer with respect to
the antenna.

FINE CALIBRATION:

More precise calibration can be obtained by using
a signal source from the sky, such as sun noise or a
signal from a satellite. For example, software can
calculate the elevation of the sun at a given time. By
peaking the noise in the receiver at that time and
comparing the calculated elevation with the inclinometer
readout the amount of error can be determined. A small
addition or subtraction to the readout can then be made.
This method has the additional value of not only
compensating for the physical position of the antenna but
also for any errors caused if the antenna pattern itself
is skewed off center. After all, it is the center of the
antenna pattern we need to position accurately. Sun noise
is useful because it is broadband and is available at the
frequency we are interested in. The use of a satellite
signal will often require shifting to a different
frequency for the measurement. If the frequency change is
too great the pattern of the antenna may be different
than at the operating frequency, introducing a position
error.

ACCURACY:

The necessary position accuracy required depends
on the beamwidth of the antenna. It is hardly necessary
to point an antenna with 0.1-degree accuracy if the
antenna pattern is 30 degrees wide. However, a 10-foot
parabolic antenna operating at 4 GHz, for example, has a
beamwidth of only about 1.8 degrees and it would be
desirable to be able to position it to within 0.2-degrees
or so.